The therapeutic efficacy of most anticancer agents is predicated on achieving adequate local delivery to the tumor site. Many cancer chemotherapeutic agents have been shown to be highly effective in vitro but not as effective in vivo. This disparity is believed to be attributable to, in part, the difficulty in delivering drug to the tumor site at therapeutic levels and the need for almost 100% cell kill to affect a cure (Jain 1994; Tannock 1998). Therapeutic molecules, cytokines, antibodies, and viral vectors are often limited in their ability to affect the tumor because of difficulty crossing the vascular wall (Yuan 1998). Inadequate specific delivery can lead to the frequently low therapeutic index seen with current cancer chemotherapeutics. This translates into significant systemic toxicities attributable to the wide dissemination and nonspecific action of many of these compounds.
Another problem is the solubility of some of the potent chemotherapeutic agents in suitable pharmaceutically acceptable vehicle for administration. Two classes of molecules widely used in chemotherapy are microtubule inhibitors such as taxane derivatives and topoisomerase I inhibitors such as camptothecin derivatives. However, it is now known as a fact that these two important classes of drugs have been formulated in vehicles which are very toxic to humans. The present invention is set to disclose novel pharmaceutical compositions to overcome the solubility and the vehicle toxicity problem of water insoluble pharmaceutical substances.
Microtuble Inhibitors as Therapeutic Agents:
Paclitaxel (Taxol, FIG. 1) is a natural diterpene product isolated from the pacific yew tree (Taxus brevifolia) by Wani et al (1971). The taxanes, paclitaxel and docetaxel (U.S. Pat. No. 4,814,470), belong to a novel class of anticancer drugs that stabilize microtubules and lead to tumor cell death. Paclitaxel (Taxol®, Bristol-Myers Squibb Co., NJ, USA), the first microtubule stabilizer identified, has proved to be of great value for the treatment of many types of cancer (Rowinsky 2001, Holton 1995). The clinical successes of paclitaxel led to the development of a second-generation taxane, docetaxel (Taxotere®, Sanofi-Aventis Pharmaceuticals, NJ, USA), and initiated the intense search for other compounds with a similar mechanism of action. Several classes of structurally diverse microtubule-stabilizing compounds have been identified. The nontaxane stabilizers identified, the epothilones (Bollag 1995), Taccalonolides (Tinley 2003) and discodermolide (Mooberry 2004 and 1999; Martello 2000), had excellent preclinical activities and are being evaluated in clinical trials as anticancer agents.
Microtubules are tubulin polymers involved in many cellular functions (Dustin 1984), one of which being the formation of the mitotic spindle required for chromosome moving to the poles of the new forming cells during cell division (Avila 1990). The importance of microtubules to cellular functions makes them a sensitive target for biological microtubule poisons. All compounds that interact with microtubules in the sense of their stabilization or disorganization are called microtubule inhibitors. They have cytotoxic effect and may kill the cell. Since microtubules are required to carry out mitosis in cell proliferation, microtubule inhibitors would primarily attack cancer cell which divides more frequently than healthy cell. Therefore many of them are very important anti-cancer compounds.
Tubulin is a protein whose quaternary structure is composed of two polypeptide subunits, α- and β-tubulin. Several isotypes have been described for each subunit in higher eucaryots. Microtubule functions are based on their capacity to polymerize and to depolymerize. This process is a very dynamic and is attend with rapid shortening or elongation of this cell structures. Tubulin is a GTP-binding protein and the binding of this nucleotide to the protein is required for microtubule polymerization, whereas the hydrolysis of the GTP bound to polymerized tubulin is required for microtubule depolymerization. Microtubule stability in healthy cell is regulated by the presence of some proteins called microtubule-associated proteins (MAP) which facilitate microtubule stabilization. The cellular mechanisms regulating microtubule assembly is highly sensitive to the concentration of Ca2+. The low cytosolic Ca2+ level characteristic of the resting state of most eucaryotic cells promotes microtubule assembly, while the localized increase in Ca2+ cause microtubule disassembly (Gelford 1991). Microtubules form through polymerization of protein dimers, consisting of one molecule each of α- and β-tubulin. Dimer and polymer are in a state of dynamic equilibrium, so that the network can respond flexibly and quickly to functional requirements. The polymer forms a fine, unbranched cylinder, usually with internal and external diameters of 14 and 28 nm, respectively, the so called microtubule (FIG. 2; Kingston 2001). Assembly is initiated by the binding together of α,β-dimers to form short protofilaments, 13 of which subsequently arrange themselves side by side to form the microtubule. Subsequent growth of the microtubule is polar, occurring mainly at the so-called plus end of the protofilaments through the addition of further dimers. Addition involves GTP, which is bound to the dimer, being cleaved to GDP, which remains attached to the tubulin. The binding site for GTP is on the b-subunit. When the cell becomes enriched with GTP-tubulin dimers, hydrolysis to GDP-tubulin falls behind the rate of assembly and a α,β-tubulin-GTP cap forms at the plus end of the protofilaments blocking further growth of the microtubule.
Microtubule inhibitors represents chemically very variegated group of compounds from different biological sources with strong effect on cytoskeletal functions and strong toxicity. Microtubule functions in cell depend on the capacity of tubulin to polymerize or the capacity of microtubules to depolymerize. Compounds which are able to influent these processes, i.e. microtubule inhibitors (also anti-tubulin agents, antimitotic agents, etc.), can be divided into four group according to their mechanism of action. 1. Compounds which bind to GTP site; 2. compounds which bind to colchicine site; 3. compounds which influence as microtubule-stabilizing agents; and 4. Compounds which do microtubule network disorganization.
In the structure of taxol there are two aromatic rings and a tetracyclic-structure containing an oxetane ring which is required for the activity of the drug. The primary action of this compound is to stabilize microtubules, preventing their depolymerization (FIG. 2). In this way taxol should block proliferating cells between G2 and mitosis, during the cell cycle. The binding of taxol appears to occur at different localizations at the amino terminal of β-tubulin, but binding to the middle region of an α-tubulin has also been reported (Loeb 1997).
A new class of microtubule-stabilizing compounds has been isolated from the bacterium Sorangium cellulosum. These macrolide compounds were called epothilones (FIG. 3), because their typical structural units are epoxide, thiazole, and ketone (Kowalski 1997; Schinzer 1996). Epothilone occurs in two structural variations, epothilone A and epothilone B, the latter containing an additional methyl group (Hyfle 1996). Epothilone A is the main product of bacteria metabolism, the yield of epothilone B amounting to 20-30 percent of the yield of epothilone A. Despite the small different in chemical structure, in most test systems epothilone B has been approximately ten-time more effective. These compounds show a striking effect on stabilizing polymerization of microtubules and they are easily obtained on large scale by a fermentation process (Gerth 1994). Both epothilones show a very narrow spectrum of activity and halts cells, as does taxol, in the G2-M phase. The Total synthesis of epothilones was reported in many laboratories (Balog 1996; Su 1997; Yang 1997; Schinzer 1997).
Interesting semisynthetic analogue of taxol with clinical use is docetaxel (Taxotere; FIG. 2), compound which contains a taxane ring linked to an oxetan ring at positions C-4 and C-5 and to an ester side chain at C-13.
Despite its broad clinical utility, there has been difficulty formulating paclitaxel and docetaxel because of their insolubility in water. The aqueous solubility of paclitaxel is only about 12 mg/liter. Paclitaxel and docetaxel are also insoluble in most pharmaceutically-acceptable solvents, and lack a suitable chemical functionality for formation of a more soluble salt. Consequently, special formulations are required for parenteral administration of paclitaxel and docetaxel. Paclitaxel and docetaxel are very poorly absorbed when administered orally (less than 1%). No oral formulation of paclitaxel has obtained regulatory approval for administration to patients.
Paclitaxel is currently formulated as Taxol®, which is a concentrated nonaqueous solution containing 6 mg paclitaxel per mL in a vehicle composed of 527 mg of polyoxyethylated castor oil (Cremophor® EL) and 49.7% (v/v) dehydrated ethyl alcohol, USP, per milliliter (available from Bristol-Myers Squibb Co., NJ, USA). Cremophor EL improves the physical stability of the solution, and ethyl alcohol solubilizes paclitaxel. The solution is stored under refrigeration and diluted just before use in 5% dextrose or 0.9% saline. Intravenous infusions of paclitaxel are generally prepared for patient administration within the concentration range of 0.3 to 1.2 mg/mL. In addition to paclitaxel, the diluted solution for administration consists of up to 10% ethanol, up to 10% Cremophor EL and up to 80% aqueous solution. However, dilution to certain concentrations may produce a supersaturated solution that could precipitate. An inline 0.22 micron filter is used during Taxol® administration to guard against the potentially life-threatening infusion of particulates.
Docetaxel is currently formulated as Taxotere®, which is a concentrated nonaqueous solution containing 40 mg docetaxel per mL in a vehicle composed of 1040 mg of polysorbate 80 and is diluted with 13% (v/v) dehydrated ethyl alcohol in water for injection (available from Sanofi-Aventis Pharmaceuticals Inc., NJ, USA). The first stage-diluted solution is further diluted just before use in 5% dextrose or 0.9% saline. Intravenous infusions of docetaxel are generally prepared for patient administration within the concentration range of 0.3 and 0.74 mg/mL. However, dilution to certain concentrations may produce a supersaturated solution that could crystallize and precipitate. An inline 0.22 micron filter is used during Taxotere® administration to guard against the potentially life-threatening infusion of particulates.
Several toxic side effects have resulted from the administration of docetaxel in the Taxotere® formulations including anaphylactic reactions, hypotension, angioedema, urticaria, peripheral neuropathy, arthralgia, mucositis, nausea, vomiting, alopecia, alcohol poisoning, respiratory distress such as dyspnea, cardiovascular irregularities, flu-like symptoms such as myalgia, gastrointestinal distress, hematologic complications such as neutropenia, genitourinary effects, and skin rashes. Some of these undesirable adverse effects were encountered in clinical trials, and in some cases, the reaction was fatal. To reduce the incidence and severity of these reactions, patients are pre-medicated with corticosteroids, diphenhydramine, H2-antagonists, antihistamines, or granulocyte colony-stimulating factor (G-CSF), and the duration of the infusion has been prolonged. Although such pre-medication has reduced the incidence of serious hypersensitivity reactions to less than 5%, milder reactions are still reported in approximately 30% of patients. All patients treated with Taxotere® are required to be pre-medicated with oral corticosteroids, such as dexamethasone 16 mg per day for 3 days starting 1 day prior to Taxotere® administration, to reduce the incidence and severity of fluid retention as well as the severity of hypersensitivity reactions
Different strategies have been pursued to produce safer and better-tolerated taxane compositions than the current ones. Alternative formulations of paclitaxel and docetaxel that avoid the use of Cremophor and polysorbate 80 have been proposed.
Phospholipid-based liposome formulations for paclitaxel, docetaxel, and other active taxanes have been developed (Sharma et al. 1993; Sharma and Straubinger 1994), and the physical properties of these and other taxane formulations have been studied (Sharma and Straubinger 1994; Balasubramanian 1994; Balasubramanian 1994). The main utility of these formulations is the elimination of toxicity related to the Cremophor EL excipient, and a reduction in the toxicity of the taxane itself, as demonstrated in several animal tumor models (Sharma 1993; A. Sharma 1995; Sharma 1996). This observation holds for several taxanes in addition to paclitaxel (Sharma 1995). In some cases, the antitumor potency of the drug appears to be slightly greater for the liposome-based formulations (Sharma 1993).
U.S. Pat. No. 6,348,215 discloses a method of stabilizing a taxane in a dispersed system, which method comprises exposing the taxane to a molecule which improves physical stability of the taxane in the dispersed system. By improving the physical stability of the taxane in the dispersed system, higher taxane content can be achieved. The patent provides a stable taxane-containing liposome preparation comprising a liposome containing one or more taxanes present in the liposome in an amount of less than 20 mol % total taxane to liposome, wherein the liposome is suspended in a glycerol:water composition having at least 30% glycerol.
U.S. Pat. Nos. 5,439,686, 5,560,933 and 5,916,596 disclose compositions for the in vivo delivery of substantially water insoluble pharmacologically active substances (such as the anticancer drug taxol) in which the pharmacologically active agent is delivered in a soluble form or in the form of suspended particles. In particular, the soluble form may comprise a solution of pharmacologically active agent in a biocompatible dispersing agent contained within a protein walled shell. Alternatively, the protein walled shell may contain particles of taxol. The polymeric shell is a biocompatible polymer, such as albumin, cross-linked by the presence of disulfide bonds. The polymeric shell, containing substantially water insoluble pharmacologically active substances therein, is then suspended in a biocompatible aqueous liquid for administration. The process for making such a polymeric shell is by emulsification of the drug alone dissolved in a nonpolar solvent such as chloroform and an aqueous solution of albumin and rapidly evaporating the emulsion around 50° C. According to the patents the process is producing cross-linked polymeric protein shell of albumin by the formation of disulfide bonds between albumin molecules and the drug is inside the polymeric shell as in a container. Further the patents distinguish the invention from protein microspheres formed by chemical cross linking and heat denaturation methods due to the formation of specific disulfide bonds with minimal denaturation of the protein. In addition, particles of substantially water insoluble pharmacologically active substances contained within the polymeric shell differ from cross-linked or heat denatured protein microspheres of the prior art because the polymeric shell produced by the process is relatively thin compared to the diameter of the coated particle.
However, it is known in the prior art that in oil-in-water emulsion using protein as emulsifying agent, certain amount of the protein may be denatured due to the interaction of the protein with the interface region between oil and water and the denatured protein may aggregate to form larger particle size due to the lower solubility of denatured protein as compared to native protein (Hegg 1982). The rest of the protein would stay in the aqueous phase as monomer. This can be demonstrated by the fact that the rapid evaporation of an oil-in-water microemulsion made by homogenization of chloroform in 2-5% albumin solution produce a hazy protein solution after evaporation around 50° C. and more than 95% of the protein is present in the solution either as monomer or dimer as measured by particle size analyzer. In other words, the protein can be recovered in a soluble form without any appreciable cross linking. Further it has been shown that disulfide cross-linking is not a determining factor in the gel formation of globular proteins (Hegg 1982) and molecular aggregations at the interface are important for emulsion stability (Dimitrova 2001). Thus the U.S. Pat. No. 5,439,686 may refer the formation of amorphous taxol nanoparticles surrounded by albumin molecules on the surface as encapsulated taxol in a protein polymeric shell formed by cross linking of the —SH groups in the protein.
Further, according to the U.S. Pat. No. 5,439,686 and U.S. Pat. No. 5,916,596, unlike conventional methods for nanoparticle formation, a polymer (e.g. polylactic acid) is not dissolved in the oil phase. The oil phase employed in the preparation of the disclosed compositions contains only the pharmacologically active agent dissolved in solvent. This is important because the U.S. Pat. No. 5,439,686 and U.S. Pat. No. 5,916,596 focused exclusively dissolving only the pharmacologically active agent and nothing else in the oil phase.
Using the technology disclosed by U.S. Pat. No. 5,439,686, a commercially viable paclitaxel formulation has been made and has been approved by the FDA for human use in 2005. It is marketed as ABRAXANE® (American Pharmaceuticals Partners Inc., IL, USA). The product description claims that ABRAXANE® for Injectable Suspension (paclitaxel protein-bound particles for injectable suspension) is an albumin-bound form of paclitaxel with a mean particle size of approximately 130 nanometers. ABRAXANE® is supplied as a white to yellow, sterile, lyophilized powder for reconstitution with 20 mL of 0.9% Sodium Chloride Injection, USP prior to intravenous infusion. Each single-use vial contains 100 mg of paclitaxel and approximately 900 mg of human albumin. Each milliliter (mL) of reconstituted suspension contains 5 mg paclitaxel. ABRAXANE® is free of solvents.
While the technology disclosed in the U.S. Pat. No. 5,439,686 is highly useful for drug delivery, it produces amorphous nanoparticles of the substantially water-insoluble pharmaceutical agent alone suspended in a protein solution. Since there is no other stabilizing forces between molecules of the substantially water-insoluble agent in the amorphous particle state except weak van der Waals interactions between them, they are prone to instability such as Ostwald ripening, since the dissolution of the amorphous particles are determined mainly by the solubility of the compound in the amorphous particles in a given medium.
Indeed, when the method described in U.S. Pat. No. 5,439,686 to produce nanoparticle dispersion was applied to produce docetaxel nanoparticle dispersion, the particles began to precipitate within 1 hour of the preparation due to Ostwald ripening. Thus the method disclosed in U.S. Pat. Nos. 5,439,686 and 5,916,596 for producing nanoparticle dispersion is not useful for the preparation of certain substantially water-insoluble pharmaceutical agents such as docetaxel nanoparticles dispersed in aqueous medium and there is a need for a new process to make stable nanoparticle dispersion of substantially water-insoluble pharmaceutical agents in aqueous solution.
US patent application 20040247660 discloses compositions and methods for protein stabilized liposomes, the creation of protein stabilized liposomes, and the administration of protein stabilized liposomes. The process involves the use of oil-in water emulsion using protein as stabilizers for the preparation of liposomes using solvent evaporation technique and produces liposomes with different physical characteristics than the solid amorphous nanoparticles disclosed in the present invention.
US patent application 20050009908 discloses a process for the preparation of a stable dispersion of solid particles, in an aqueous medium comprising combining (a) a first solution comprising a substantially water-insoluble substance, a water-miscible organic solvent and an inhibitor with (b) an aqueous phase comprising water and optionally a stabiliser, thereby precipitating solid particles comprising the inhibitor and the substantially water-insoluble substance; and optionally removing the water-miscible organic solvent; wherein the inhibitor is a non-polymeric hydrophobic organic compound as defined in the description. The process provides a dispersion of solid particles in an aqueous medium, which particles exhibit reduced particle growth mediated by Ostwald ripening. The application describes the preparation of nanoparticles through precipitation technique using water miscible organic solvents. The problem with the method is to control the size of the particle as it is difficult to control the particle size through precipitation technique. This method is entirely different from the present invention wherein water immiscible organic solvent is used to form fine oil-in water emulsion and subsequent evaporation of water immiscible organic solvent to form nano-particles.
US application 20060141043A discloses the preparation of a stable dispersion of solid particles, in an aqueous medium comprising combining (a) a first solution comprising a substantially water-insoluble substance which is a thiazole compound, a water-miscible organic solvent and an inhibitor with (b) an aqueous phase comprising water and optionally a stabiliser, thereby precipitating solid particles comprising the inhibitor and the substantially water-insoluble substance; and optionally removing the water-miscible organic solvent; wherein the inhibitor is a non-polymeric hydrophobic organic compound.
Method for Nanoparticle Preparation:
There are several methods disclosed in the literature for the preparation of solid nanoparticles. For example, solid lipid nanoparticles (SLN) are nanoparticles with a matrix being composed of a solid lipid, i.e. the lipid is solid at room temperature and also at body temperature (Muller et al., 1995; Lucks and Muller, 1996; Muller et al., 2000; Mehnert and Mader, 2001). The lipid is melted approximately 5° C. above its melting point and the drug dissolved or dispersed in the melted lipid. Subsequently, the melt is dispersed in a hot surfactant solution by high speed stirring. The coarse emulsion obtained is homogenised in a high-pressure unit, typically at 500 bar and three homogenisation cycles. A hot oil-in-water nanoemulsion is obtained, cooled, the lipid recrystallises and forms solid lipid nanoparticles. Identical to the drug nanocrystals the SLN possess adhesive properties. They adhere to the gut wall and release the drug exactly where it should be absorbed. In addition the lipids are known to have absorption promoting properties, not only for lipophilic drugs such as Vitamin E but also drugs in general (Porter and Charman, 2001; Sek et al., 2001; Charman, 2000). There are even differences in the lipid absorption enhancement depending on the structure of the lipids (Sek et al., 2002). Basically, the body is taking up the lipid and the solubilised drug at the same time.
Meanwhile the second generation of lipid nanoparticles with solid matrix has been developed, the so-called nanostructured lipid carriers (Muller et al., 2000b). The NLC® are characterised that a certain nanostructure is given to their particle matrix by preparing the lipid matrix from a blend of a solid lipid with a liquid lipid (oil). The mixture is still solid at 40° C. These particles have improved properties regarding payload of drugs, more flexibility in modulating the drug release profile and being also suitable to trigger drug release (Muller et al., 2002; Radtke and Muller, 2001b). They can also be used for oral and parenteral drug administration identical to SLN, but have some additional interesting features.
In the LDC® nanoparticle technology (Muller and Olbrich, 1999b, 2000b, 2000c), the “conjugates” (term used in its broadest sense) were prepared either by salt formation (e.g. amino group containing molecule with fatty acid) or alternatively by covalent linkage (e.g. ether, ester, e.g. tributyrin). Most of the lipid conjugates melt somewhere about approximately 50-100° C. The conjugates are melted and dispersed in a hot surfactant solution. Further processing was performed identical to SLN and NLC. The obtained emulsion system is homogenised by high-pressure homogenisation, the obtained nanodispersion cooled, the conjugate recrystallises and forms LDC nanoparticles. One could consider this suspension also as a nanosuspension of a pro-drug.
U.S. Pat. No. 6,207,178 discloses the preparation of suspensions of colloidal solid lipid particles (SLPs) of predominantly anisometrical shape with the lipid matrix being in a stable polymorphic modification and of suspensions of micron and submicron particles of bioactive agents (PBAs). A suspension stable for at least about 12 months of particles of bioactive agents (PBAs) manufactured by an emulsifying process having the following steps: (a) melting at least one solid bioactive agent; (b) heating a dispersion medium to approximately the same temperature as said at least one molten solid bioactive agent formed by step (a); (c) adding at least one highly mobile water-soluble or dispersible stabilizer, which does not form a separate phase in the dispersion medium, to the dispersion medium in an amount effective after emulsification to stabilize newly created surfaces during recrystallization, and optionally adding at least one lipid-soluble or dispersible stabilizer to said at least one molten bioactive agent; (d) premixing said at least one molten bioactive agent and the dispersion medium, and subsequently homogenizing said mixture by high pressure homogenization, micro-fluidization and/or ultrasonication; and (e) allowing the homogenized dispersion to cool until solid particles are formed by recrystallization of the dispersed bioactive agents.
One of the problem of applying these techniques for the preparation of solid nanoparticles containing taxanes are the fact that some of the taxanes such as docetaxel are prone to decomposition at high temperatures as used in these techniques. Another disadvantage is the formation of crystalline nanoparticles which may affect the stability and release characteristics of the encapsulated drug.
Another common method for the preparation of solid nanoparticles is by the solvent evaporation of an oil-in-water emulsion. The oil-phase contains one or more pharmaceutical substances and the aqueous phase contains just the buffering materials or an emulsifier. An emulsion consists of two immiscible liquids (usually oil and water), with one of the liquids dispersed as small spherical droplets in the other. In most foods, for example, the diameters of the droplets usually lie somewhere between 0.1 and 100 μm (Dickinson and Stainsby 1982, Dickinson 1992). An emulsion can be conveniently classified according to the distribution of the oil and aqueous phases. A system that consists of oil droplets dispersed in an aqueous phase is called an oil-in-water or O/W emulsion (e.g, mayonnaise, milk, cream etc.). A system that consists of water droplets dispersed in an oil phase is called a water-in-oil or W/O emulsion (e.g. margarine, butter and spreads). The process of converting two separate immiscible liquids into an emulsion, or of reducing the size of the droplets in a preexisting emulsion, is known as homogenization.
It is possible to form an emulsion by homogenizing pure oil and pure water together, but the two phases rapidly separate into a system that consists of a layer of oil (lower density) on top of a layer of water (higher density). This is because droplets tend to merge with their neighbors, which eventually leads to complete phase separation. Emulsions usually are thermodynamically unstable systems. It is possible to form emulsions that are kinetically stable (metastable) for a reasonable period of time (a few minutes, hours, days, weeks, months, or years) by including substances known as emulsifiers and/or thickening agent prior to homogenization.
Emulsifiers are surface-active molecules that adsorb to the surface of freshly formed droplets during homogenization, forming a protective membrane that prevents the droplets from coming close enough together to aggregate. Most emulsifiers are molecules having polar and nonpolar regions in the same molecule. The most common emulsifiers used in the food industry are amphiphilic proteins, small-molecule surfactants, and monoglycerides, such as sucrose esters of fatty acids, citric acid esters of monodiglycerides, salts of fatty acids, etc (Krog, 1990).
Thickening agents are ingredients that are used to increase the viscosity of the continuous phase of emulsions and they enhance emulsion stability by retarding the movement of the droplets. A stabilizer is any ingredient that can be used to enhance the stability of an emulsion and may therefore be either an emulsifier or thickening agent.
The term “emulsion stability” is broadly used to describe the ability of an emulsion to resist changes in its properties with time. Emulsions may become unstable through a variety of physical processes including creaming, sedimentation, flocculation, coalescence, and phase inversion. Creaming and sedimentation are both forms of gravitational separation. Creaming describes the upward movement of droplets due to the fact that they have a lower density than the surrounding liquid, whereas sedimentation describes the downward movement of droplets due to the fact that they have a higher density than the surrounding liquid. Flocculation and coalescence are both types of droplet aggregation. Flocculation occurs when two or more droplets come together to form an aggregate in which the droplets retain their individual integrity, whereas coalescence is the process where two or more droplets merge together to form a single larger droplet. Extensive droplet coalescence can eventually lead to the formation of a separate layer of oil on top of a sample, which is known as “oiling off”.
Most emulsions can conveniently be considered to consist of three regions that have different physicochemical properties: the interior of the droplets, the continuous phase, and the interface. The molecules in an emulsion distribute themselves among these three regions according to their concentration and polarity (Wedzicha 1988). Nonpolar molecules tend to be located primarily in the oil phase, polar molecules in the aqueous phase, and amphiphilic molecules at the interface. It should be noted that even at equilibrium, there is a continuous exchange of molecules between the different regions, which occurs at a rate that depends on the mass transport of the molecules through the system. Molecules may also move from one region to another when there is some alteration in the environmental conditions of an emulsion (e.g, a change in temperature or dilution within the mouth). The location and mass transport of the molecules within an emulsion have a significant influence on the aroma, flavor release, texture, and physicochemical stability of food products (Dickinson 1982; Wedzicha 1991; Coupland 1996).
Many properties of the emulsions can only be understood with reference to their dynamic nature. The formation of emulsions by homogenization is a highly dynamic process which involves the violent disruption of droplets and the rapid movement of surface-active molecules from the bulk liquids to the interfacial region. Even after their formation, the droplets in an emulsion are in continual motion and frequently collide with one another because of their Brownian motion, gravity, or applied mechanical forces (Melik 1988; Dukhin 1996). The continual movement and interactions of droplets cause the properties of emulsions to evolve over time due to the various destabilization processes such as change in temperature or in time.
The most important properties of emulsion are determined by the size of the droplets they contain (Dickinson 1982; 1992). Consequently, it is important to control, predict and measure, the size of the droplets in emulsions. If all the droplets in an emulsion are of the same size, the emulsion is referred to as monodisperse, but if there is a range of sizes present, the emulsion is referred to as polydisperse. The size of the droplets in a monodisperse emulsion can be completely characterized by a single number, such as the droplet diameter (d) or radius (r). Monodisperse emulsions are sometimes used for fundamental studies because the interpretation of experimental measurements is much simpler than that of polydisperse emulsions. Nevertheless, emulsions by homogenization always contain a distribution of droplet sizes, and so the specification of their droplet size is more complicated than that of monodisperse systems. Ideally, one would like to have information about the full particle size distribution of an emulsion (i.e, the size of each of the droplets in the system). In many situations, knowledge of the average size of the droplets and the width of the distribution is sufficient (Hunter 1986).
An efficient emulsifier produces an emulsion in which there is no visible separation of the oil and water phases over time. Phase separation may not become visible to the human eye for a long time, even though some emulsion breakdown has occurred. A more quantitative method of determining emulsifier efficiency is to measure the change in the particle size distribution of an emulsion with time. An efficient emulsifier produces emulsions in which the particle size distribution does not change over time, whereas a poor emulsifier produces emulsions in which the particle size increases due to coalescence and/or flocculation. The kinetics of emulsion stability can be established by measuring the rate at which the particle size increases with time.
Proteins as Emulsifiers:
In oil-in-water emulsions, proteins are used mostly as surface active agents and emulsifiers. One of the food proteins used in o/w emulsions is whey proteins. The whey proteins include four proteins: β-lactoglobulin, α-lactalbumin, bovine serum albumin and immunoglobulin (Tornberg 1990). Commercially, whey protein isolates (WPI) with isolectric point ˜5 (Tornberg 1990) are used for o/w emulsion preparation. According to Hunt (1995), whey protein concentrations of 8% have been used to produce self-supporting gels. Later on, the limiting concentrations of whey protein to produce self-supporting gels are known to be reduced to 4-5%. It is possible to produce gels at whey protein concentrations as low as 2% w/w, using heat treatments at 90° C. or 121° C. and ionic strength in excess of 50 mM (Hunt 1995).
U.S. Pat. No. 6,106,855 discloses a method for preparing stable oil-in-water emulsions by mixing oil, water and an insoluble protein at high shear. By varying the amount of insoluble protein the emulsions may be made liquid, semisolid or solid. The preferred insoluble proteins are insoluble fibrous proteins such as collagen. The emulsions may be medicated with hydrophilic or hydrophobic pharmacologically active agents and are useful as or in wound dressings or ointments.
U.S. Pat. No. 6,616,917 discloses an invention relating to a transparent or translucent cosmetic emulsion comprising an aqueous phase, a fatty phase and a surfactant, the said fatty phase containing a miscible mixture of at least one cosmetic oil and of at least one volatile fluoro compound, the latter compound being present in a proportion such that the refractive index of the fatty phase is equal to ±0.05 of that of the aqueous phase. The invention also relates to the process for preparing the emulsion and the use of the emulsion in skincare, hair conditioning and antisun protection and/or artificial tanning.
Proteins derived from whey are widely used as emulsifiers (Phillips 1994; Dalgleish 1996). They adsorb to the surface of oil droplets during homogenization and form a protective membrane, which prevents droplets from coalescing (Dickinson 1998). The physicochemical properties of emulsions stabilized by whey protein isolates (WPI) are related to the aqueous phase composition (e.g, ionic strength and pH) and the processing and storage conditions of the product (e.g, heating, cooling, and mechanical agitation). Emulsions are prone to flocculation around the isoelectric point of the WPI, but are stable at higher or lower pH (Philips 1994). The stability to flocculation could be interpreted in terms of colloidal interactions between droplets, i.e, van der Waals, electrostatic repulsion and steric forces (Philips 1994; Dalgleish 1996). The van der Waals interactions are fairly short-range due to their dependence on the inverse 6th power of the distance. Electrostatic interactions between similarly charged droplets are repulsive, and their magnitude and range decrease with increasing ionic strength. Short range interactions become important at droplet separations of the order of the thickness of the interfacial layer or less, e.g, steric, thermal fluctuation and hydration forces (Israelachvili 1992). Such interactions are negligible at distances greater than the thickness of the interfacial layer, but become strongly repulsive when the layers overlap, preventing droplets from getting closer. It has been shown that the criteria for the protein emulsifiers appear to be the ability to adsorb quickly at the oil/water interface and surface hydrophobicity is of secondary importance (Lockhead 1999).
Thus, in the preparation of nanoparticle using solvent evaporation technique, proteins can be used as emulsifier to form the fine oil-in-water emulsion and subsequently the organic solvent in the emulsion can be evaporated to form the nanoparticles. Human serum albumin can be ideal for such preparations as it is non-immunogenic in humans, has the desired property as an emulsifier and has preferential targeting property to tumor sites. The measurements using the phosphorescence depolarization technique support a rather rigid heart shaped structure (8 nm×8 nm×3.2 nm) of albumin in neutral solution of BSA as in the crystal structure of human serum albumin (Ferrer 2001) and serum albumin has been shown to have good gelling properties (Hegg 1982).
Polymers as Emulsifiers:
Apart from proteins as emulsifiers, several natural, semi-natural and synthetic polymers can be used as emulsifiers. The polymer emulsifiers include naturally occurring emulsifiers, for example, agar, carageenan, furcellaran, tamarind seed polysaccharides, gum tare, gum karaya, pectin, xanthan gum, sodium alginate, tragacanth gum, guar gum, locust bean gum, pullulan, jellan gum, gum Arabic and various starches. Semisynthetic emulsifiers include carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxyethyl cellulose (HEC), alginic acid propylene glycol ester, chemically modified starches including soluble starches, and synthetic polymers including polyvinyl alcohol, polyethylene glycol and sodium polyacrylate. These polymer emulsifiers are used in the production of emulsion compositions such as emulsion flavors or powder compositions such as powder fats and oils and powder flavors. The powder composition is produced by emulsifying an oil, a lipophilic flavor or the like, and an aqueous component with a polymer emulsifier and then subjecting the emulsion to spray drying or the like. In this case, the powder composition is often in the form of a microcapsule.
Ostwald Ripening:
Generally, if particles with a wide range of sizes are dispersed in a medium there will be a differential rate of dissolution of the particles in the medium. The differential dissolution results in the smaller particles being thermodynamically unstable relative to the larger particles and gives rise to a flux of material from the smaller particles to the larger particles. The effect of this is that the smaller particles dissolve in the medium, whilst the dissolved material is deposited onto the larger particles thereby giving an increase in particle size. One such mechanism for particle growth is known as Ostwald ripening (Ostwald 1896 & 1897). Ostwald ripening has been studied extensively due to its importance in material and pharmaceutical sciences (Baldan 2002; Voorhees 1992; Madras 2001 & 2002; Lifshitz 1961 and Davies 1981).
The growth of particles in a dispersion can result in instability of the dispersion during storage resulting in the sedimentation of particles from the dispersion. It is particularly important that the particle size in a dispersion of a pharmacologically active compound remains constant because a change in particle size is likely to affect the bioavailability, toxicity and hence the efficacy of the compound. Furthermore, if the dispersion is required for intravenous administration, growth of the particles in the dispersion may render the dispersion unsuitable for this purpose, possibly leading to adverse or dangerous side effects.
Theoretically particle growth resulting from Ostwald ripening would be eliminated if all the particles in the dispersion were the same size. However, in practice, it is impossible to achieve a completely uniform particle size and even small differences in particle sizes can give rise to particle growth.
U.S. Pat. No. 4,826,689 describes a process for the preparation of uniform sized particles of a solid by infusing an aqueous precipitating liquid into a solution of the solid in an organic liquid under controlled conditions of temperature and infusion rate, thereby controlling the particle size. U.S. Pat. No. 4,997,454 describes a similar process in which the precipitating liquid is non-aqueous. However, when the particles have a small but finite solubility in the precipitating medium particle size growth is observed after the particles have been precipitated. To maintain a particular particle size using these processes it is necessary to isolate the particles as soon as they have been precipitated to minimise particle growth. Therefore, particles prepared according to these processes cannot be stored in a liquid medium as a dispersion. Furthermore, for some materials the rate of Ostwald ripening is so great that it is not practical to isolate small particles (especially nano-particles) from the suspension.
Higuchi and Misra (J. Pharm. Sci., 51 (1962) 459) describe a method for inhibiting the growth of the oil droplets in oil-in-water emulsions by adding a hydrophobic compound (such as hexadecane) to the oil phase of the emulsion. U.S. Pat. No. 6,074,986 (WO95/07614) describes the addition of a polymeric material having a molecular weight of up to 10,000 to the disperse oil phase of an oil-in-water emulsion to inhibit Ostwald ripening. Welin-Berger et al. (Int. Jour. of Pharmaceutics 200 (2000) pp 249-260) describe the addition of a hydrophobic material to the oil phase of an oil-in-water emulsion to inhibit Ostwald ripening of the oil droplets in the emulsion. In these latter three references the material added to the oil phase is dissolved in the oil phase to give a single phase oil dispersed in the aqueous continuous medium.
EP 589 838 describes the addition of a polymeric stabilizer to stabilize an oil-in-water emulsion wherein the disperse phase is a hydrophobic pesticide dissolved in a hydrophobic solvent.
U.S. Pat. No. 4,348,385 discloses a dispersion of a solid pesticide in an organic solvent to which is added an ionic dispersant to control Ostwald ripening.
WO 99/04766 describes a process for preparing vesicular nano-capsules by forming an oil-in-water emulsion wherein the dispersed oil phase comprises a material designed to form a nano-capsule envelope, an organic solvent and optionally an active ingredient. After formation of a stable emulsion the solvent is extracted to leave a dispersion of nano-capsules.
U.S. Pat. No. 5,100,591 describes a process in which particles comprising a complex between a water insoluble substance and a phospholipid are prepared by co-precipitation of the substance and phospholipid into an aqueous medium. Generally the molar ratio of phospholipid to substance is 1:1 to ensure that a complex is formed.
U.S. Pat. No. 4,610,868 describes lipid matrix carriers in which particles of a substance is dispersed in a lipid matrix. The major phase of the lipid matrix carrier comprises a hydrophobic lipid material such as a phospholipid.
There has been no recognition or appreciation in the art that prior to this application that a substantially stable nanoparticle by inhibiting the Ostwald ripening can be formed by the solvent evaporation of an oil-in-water emulsion using protein such as serum albumin or a polymer such as polyvinyl alcohol as emulsifying agent. The present invention discloses a new drug delivery system for the delivery of substantially water insoluble pharmaceutical substances selected as nanoparticles without appreciable Ostwald ripening effect for the treatment of diseases in humans with reduced toxicity.